In recent years, separation of water-in-oil emulsions has become industrially important. Water present in liquid fuels can combine with chemicals in fuels, such as sulfur, to form corrosive compounds which can damage sensitive engine parts. Corrosion is a major cause of reduction of engine life and efficiency.
In addition, surfactants present in liquid fuels lower the interfacial tension between water and fuel and the problem of separation becomes more insidious. This can also cause a product to be off-specification due to haze and color [1, 2]. The water is present in fuels as primary emulsions with drop sizes greater than 100 μm and as secondary emulsions with drop sizes less than 100 μm. The separation of primary emulsions is often accomplished by gravity settling or mechanical means. However, coalescence filtration using fibrous filters is an efficient and economical way to separate secondary dispersions.
The coalescence process occurs in three main steps. First, the fibrous bed captures water droplets. Second, the collected water phase migrates through the bed and coalesces. Third, the enlarged droplets are released from the fiber surfaces [3]. Coalescer performance is generally characterized by separation efficiency and pressure drop. The separation efficiency is highly dependent on the characteristic properties of the dispersions (e.g. composition, density, viscosity drop size) and the fiber bed (material, diameter, surface structure, porosity) [4]. Flow rate is an important factor in water-in-oil dispersion flow, as it controls the capture mechanism and capture probability of droplets and the distribution of the dispersed phase. [3].
Shin [3] shows that wettability of the surface has effect on drop attachment on silane coated glass rods. It is known that critical surface tension of a solid to the liquid surface tension determines the character of solid wettability [5]. Wettability of filter medium can be represented by its hydrophobic or hydrophilic behavior. Moorthy [6] performed the coalescence experiments with surface functionalized filter media and showed that intermediate wettability gives better performance.
Research results and common experience in industry show that coalescing filters work best with an intermediate wettability. If the medium is too wetting it tends to load up with liquid drops which restricts the air flow and results in an undesired high pressure drop. If the medium is too non-wetting the droplets captured on the fibers quickly move along the fibers or leave the fibers before they have a chance to coalesce and hence the separation is not efficient.
Common practice to control the wettability is by selecting a material from which the fibers are made that has an intermediate wettability, or by applying a coating, such as silanes, that gives the surface of the fiber structures the intermediate wettability. The difficulty here is finding the right material or coating that has the best wettability for a given application. This approach does not allow for incremental changes in wettability.
The above-noted references as well as others are as follows:    1. Lloyd A. Spielman et al., “A review of progress in the coalescence of liquid—liquid suspension and a new theoretical framework for Coalescence by porous media are presented” Industrial and Engineering Chemistry, Vol. 62, No. 10, 10-24 October 1970.    2. Improve suspended water removal from fuels: A better understanding of molecular forces enhances free water separator selection R. L. Brown, Jr., et al., Pall Corp., East Hills, N.Y. from Hydrocarbon Processing®, December 1993.    3. C. Shin and G. G. Chase “The effect of wettability on drop attachment to glass rods”, Journal of Fluid Particle Separation, Vol. 16, No. 1, 1-7, 2004.    4. Hauke Speth., et al., “Coalescence of secondary dispersions in fiber beds”, Separation and Purification Technology, Vol. 29, 113-119, 2002.    5. Secerov Sokolovic, et al., “Effect of the Nature of Different Polymeric Fibers on Steady-State Bed Coalescence of an Oil-in-Water Emulsion”, Industrial & Engineering Chemistry Research Vol. 43 (20), 6490-6495, 2004.    6. K. Moorthy, et. al., “Effect of Wettability on liquid -liquid coalescence”, AFS Conference Ann Harbor, September 2005.    7. Erbil H. Y., et al., “Transformation of Simple surface into super-hydrophobic surface”, Science, Vol. 299, 1377-1380, 2003.    8. Washburn E. W, “The dynamics of capillary flow”, The American Physical Society, VOX-V II, No. 3, 374-375.    9. Murata Toshiaki et al., “A modified penetration rate for measuring the wettability of Coal Powders”, Journal of Japan Oil and Chemists Society, Vol. 32 (9), 498-502, 1983.    10. Kondo, Hiroshi, et al.,“Lipid compounds in the sediment cores of Lake Kawahara Oike, pagasaki Prefecture, Japan], documenting its change from brackish water to fresh water”, Daigaku Kyoikugakubu Shizen Kagaku Kenkyu Hokoku, Vol. 49 13-25, (1993).    11. Voyutskii et al., (1953 Voyutskii, S. S, Akl'yanova, K. A., Panich, R., Fodiman, N., “Mechanism of separation of disperse phase of emulsions during filtration”, Dokl. Akad. Nauk SSSR, 91 (1953), 1155    12. Hazlett (1969) Hazlett, R. N., “Fibrous Bed coalescence of water”, I & EC Fundamentals, 8 (1969), 625    13. Clayfield et al, (1984) Clayfield, E. J, Dixon, A. G, Foulds, A. W and Miller, R. J. L, “Coalescence of secondary emulsions”, Journal of Colloid and Interface Science, 104 (1985), 498    14. Moses and Fg (1985) Moses, S. F. and Ng, K. M. (1985) A visual study of the breakdown of emulsions in porous coalescers. Chem. Eng. Sci., 40 (12): 2339-2350.    15. Basu (1993) Basu, S, “A Study on effect of wetting on mechanism of coalescence in a model coalescer”, Journal of Colloid and Interface Science, 159 (1993), 68    16. Bin Ding, et al., Conversion of an electro-sound nanofibrous cellulose acetate mat from a super-hydrophilic to super-hydrophobic surface. Nanotechnology 17 (2006) 4332-4339    17. Mane R. S, et al., A simple and low temperature process for super-hydrophilic Rutile TiO2 thin films growth, Applied Surface Science, 253 (2006) 581-585    18. Ren O, et al., Study on the Superhydrophilicity of the SiO2-TiO2 thin films prepared by sol-gel method at room temperature, J. of Sol-gel Science and Technology, 29 (2004) 13 1-136    19. Guo Z, et al., Stable bio-mimetic Super-hydrophobic engineering materials, JAGS Communications 127 (2005) 15670-15671    20. Ma Y., et al., Fabrication of super-hydrophobic film from PMMA with intrinsic contact angle below 90′. Polymer 48 (2007) 7455-7460    21. Van der wal P., et al., Super-hydrophobic surfaces made from Teflon, Soft Matter 3 (2007) 426-429    22. Feng X, et al., Reversible superhydrophobicity to super-hydrophilicity transition of aligned ZnO nano-rod films, JAGS Communication 126 (2004) 62-63    23. Ma M., et al., Electrospun Poly (Styrene-block-dimetylsiloxane) block copolymer fibers exhibiting superhydrophobicity, Langmuir, 21 (2005) 5549-5554    24. Onda T, et al., Super-water-repellent fractal surfaces, Langmuir, Vol. 12 Number 9 (1996) 2125-2127    25. Mohammadi R., et al., Effect of surfactants on wetting of super-hydrophobic surfaces, Langmuir, 20 (2004) 9657-9662    26. Zhang X., et al., A transparent and photo-patternable super-hydrophobic film, Chem. Commun (2007) 4949-4951    27. Tadanaga K., Morinaga J., Minami T., Formation of superhydrophobicsuperhydrophilic pattern on flowerlike Alumina thin film by Sol-gel method, J. of Sol-Gel Science and Technology 19 (2000) 21 1-214    28. U.S. Pat. No. 5,102,745, granted April 7, 1992 to Tatarchuk et al.